为什么函数runtime_args可以这样表示？

This code is in proc.go. I can not understand function runtime_args, can anyone help me? Sorry for my poor english.

// Args hold the command-line arguments, starting with the program name.
var Args []string

func init() {
    if runtime.GOOS == "windows" {
        // Initialized in exec_windows.go.
        return
    }
    Args = runtime_args()
}

func runtime_args() []string // in package runtime

// Getuid returns the numeric user id of the caller.
//
// On Windows, it returns -1.
func Getuid() int { return syscall.Getuid() }

According to Go Programming Language Specification:

A function declaration may omit the body. Such a declaration provides the signature for a function implemented outside Go, such as an assembly routine.

In your case this function implementation is actually declared in the runtime package

proc.go:

...
func runtime_args() []string // in package runtime
...

runtime.go:

...
//go:linkname os_runtime_args os.runtime_args
func os_runtime_args() []string { return append([]string{}, argslice...) }
...